Today, binary coded radar systems are used for both civilian and military purposes In radar systems, the Barker code is one of the most commonly used binary phase-coded waveforms, because it has a relatively high ratio between its mainlobe and sidelobes, and the length of the code is also relatively short. The maximum length of the Barker code is 13. When using the Barker code and the pulse compression technique, it is also possible to achieve a better range resolution. The invention can be applied in various radar systems, such as MTI (Moving Target Indicator), PD (Pulse-Doppler) and SAR (Synthetic Aperture Radar) systems. Barker coding waveforms are sometimes used also in spread spectrum communication.
This invention is also applicable with other binary coding waveforms, such as truncated PN sequences and concatenated codes.
When considering the Barker code as a pulse compression waveform, the sidelobe level of the Barker code after matched filtering should be suppressed to a certain low level to be able to achieve a good resolution for ranging and speed measuring. That is why the optimization of a sidelobe filter is a matter of great importance in modern radar systems. Without suppression, the peak sidelobe level for a 13 bit Barker code is only 22.3 dB lower than the mainlobe level, and this is not sufficiently low for most radar applications.
There are mainly three possible ways to design a Barker code sidelobe suppression filter. The first two methods are called LMS and LP methods In these methods, a mismatched filter is designed for the Barker code signal directly, instead of first using a matched filter to perform the pulse compression correlation and then suppressing the sidelobes later. Both the LP algorithm and the LMS algorithm are usable when synthesizing the filters in the time domain.
By the LMS (Least Mean Square) algorithm, the LMS filter is designed to replace the Barker code matched filter. The LMS mismatched filters for the Barker code are utilized to minimize the least mean square of the sidelobe, that is to say to minimize the average energy of the sidelobe. Minimizing the average energy of the sidelobe is not enough, because it does not assure that the peak sidelobe is minimized. In radar applications, a high peak sidelobe of a strong target echo can sometimes mask the mainlobe of a weak target echo. Also, the LMS sidelobe suppression filters are complex in their filter structure. The LMS technique is presented in the publication: M. H. Ackroyd and F. Ghani, Optimum Mismatched Filters for Sidelobe Suppression, IEEE Transactions on Aerospace and Electronic systems, Vol AES-9, No. 2, March 1973, pp. 214-218.
Another prior method of designing a mismatched filter for the Barker codes utilizes LP algorithm (Linear Programming). Linear Programming techniques are utilized to determine the optimal filter weights in order to minimize the peak range sidelobes of a binary phase-coded waveform such as the Barker code. The output peak sidelobes of the filters designed by the LP algorithm are lower than those of the filters designed by the LMS algorithm. Another problem with the LP technique is the complexity of the filter structure. For example, when using a 13 bit Barker code, at least 20 tapped delay elements are needed in the filter to obtain an acceptable performance. The LP technique is presented in the publication: S. Zoraster, Minimum Peak Range Sidelobe Filters for Binary Phase-Coded Waveforms, IEEE Transactions on Aerospace and Electronic Systems, Vol AES-16, No. 1, January 1980, pp. 112-115.
The third sidelobe reduction filter design method is to synthesize, in the frequency domain, a R-G filter, which is a separate sidelobe suppression filter connected to the matched filter. The hardware structure of the R-G filter is relatively simple as compared with LMS and LP filters, but its performance is not as good as that of the LP filters. It should also be pointed out that the R-G filters can only be used for the Barker codes with positive sidelobes. For example, an 11 bit Barker code has negative sidelobes, thus the R-G filters cannot be used for it. The R-G filters are presented in the publication: A. W. Rihaczek, R. M. Golden, Range Sidelobe Suppression for Barker Codes, IEEE Transactions on Aerospace and Electronic Systems, Vol AES-7, No. 6, Nov. 1971, pp. 1087-1092.